Manufacture of calcium arsenate



July 5, 1938.` A H, [-{EmuhJGER` 2,122,861

MANUFACTURE OF' CALCIUM ARSENATE Filed May 24, 1935 aNvEN-roR iff/zar l Hem/'nger ATTORNEY Patented July 5, 1938 PpIrENT oFFlcE i `.2,122,861 e MANUFACTURE OF` CALCIUBI ARSENATE Y Arthur H. Henninger, Floral Park, N. Y., assignor to General Chemical Company,

New York,

n N. Y., a `corporation of NewYork y Application May 2 4, 1935, Serial No. 23,246

" roiaim.. `(01;` 23;-53)

This invention relates to the manufacture of calcium arsenate. tion is directed to calcium arsenates lowin water soluble arsenic and especially suitable for use `as insecticides'V without burning foliage, and Vto methods for making these products.

In commercial practice,

usually made ,by forming a water slurry of calcium hydrate and adding arsenic acid in quantity sufficient to react with the calcium hydrate and form the water insoluble basic calcium arsenates. Inthis reaction; as usually carried out, there are alsopformed relatively small amounts of water soluble arsenates of calcium. It is generally acknowledged that the water soluble arsenic content of arsenical insecticides causes burning of foliage and the'amount of Water soluble arsenic is a measure of the burning effect of such insectieide.` Calcium arsenate has been used for insecticidal purposes for years, and during this time it has been the aim of producers to make calcium arsenate containing as little water soluble arsenic as possible. l stated to be the acid salt, calcium tetrahydric arsenate CaH4 AsO4 a and 25 lieved that this salt is formed during production ofand becomes occluded in thewater insoluble arsenate precipitate which constitutes the bulk of the product. arsenates maybe formed. Manydiflerent methnate `with a view ods involving addition of other vcompounds to the reaction mixture such as analkali hydroxide, or

a. halide or nitrate of alkali or alkalineearth.

Such foreign or extraneous substances either dilutethe finalI products or necessitateyremoval by treatment of the 'water insoluble calcium arsenate low in water soluble arsenic content can bepro` c acidalone without duced from lime and arseni addition of other compounds.v

that formationr of water soluble arsenic may be minimized by adding the acid tothe slurry while effecting turbulent agitation of the slurry, inthe that required to react with the calcium hydrate',

More particularly the inven'' calciumv arsenate is` The water soluble arsenic hasbeen it is commonly be-l Other water vsoluble calcium ods have been proposedfor making calcium arse to reducingthe water soluble arsenic content of theproduct, including meth-f In accordance with the present invention IlA have discovered that a `stable calcium a'rsenatef CAccording to the invention, ith'as been found 1 zone of introduction of the acid, in such a way` as to avoid any local excess of arsenic acid over j tions of acidand calcium hydroxide of such nature as to produce the Water soluble calcium arsehates are substantially avoided.

In maldng'the products of the invention, the

improved process of the invention may be carried out in apparatus one suitable lembodiment' of which is illustratedin vertical section and in plan in Figs. l and 2 of the accompanying draw' ing, the section of Fig. l being taken approximately on line "I-I of Fig. 2.

Referring to the drawing, mixing tank I0, having a brick lining Il, is of circular horizontal cross-section, and in the present example, it may be considered the diameter and the depth of the tank are approximately equal. Agitator shaft I2, journalled at the lower end in a bearing I3 on the bottom of the tank, carries a turbine agitator I4 comprising say 10 radially disposed mixer blades I5. A Vsecond similarly made agitator I4' is xed to shaft I2 and is'spaced from the bottom of tank I0 a distance equal to about a third of the depth of the tank.

Atubular hub I6 is fastened tightly to shaft I2 by clamps I1.` The cylindrical acid receiver I9 is welded at 20 to and hence carried and supported by hub I6. The top edge of the receiver is flanged inwardly to prevent escape of liquid forced by centrifugal motion against the vertical walls of the receiver. Projecting downwardly from near the outer circumference-of receiver I9 are4` several for example four liquiddelivery pipes 23. Itis noted pipes 23'are spaced outwardly from the rotating vshaft VI2 an appreciable distance so as to effect-discharge by centrifugal force of acid from the receiver and delivery pipes. The latter are held inplace by brackets 25 fixed to shaft I2. As will be seen from the drawing, the outlet ends of pipes 23 terminate in flattened surfaces 2 providing Vplane-like 4areas facilitating delivery of acid to the slurry in relatively thin films. It`will be understood the `surfaces from which acid is delivered face in the `directionof movement of f pipes 23v and receiver I9 all of which 'rotate with shaftl I2 and Aagitator I4. Acid is fedfinto receiver I9 through a stationary inlet pipe 30 supported by a bracket 3l. Four vertical baiiles 35, 36, 3l and 38 are fastened to the inner walls of the tank, preferably at intervals of about 90. There are also vprovidedtwo horizontal baffles 40 and 4I attached at either ends to oppositely disposed baffles. Y i V 'i One example oflcarrying out the method of the invention to produce a stable calcium arsnate low in Water soluble' arsenic is as follows:` About 1170' gallons of water are run intoA tank I0, with the agitators I4 and I4' running at about 125 R. P. M. About 2475 pounds hydrated lime or freshly slaked lime are added. Approximately '75% arsenic acid, equivalent to 2275 pounds 100% acid, is run into the slurry in a continuous stream, during a period of about one hour, through line 30, receiver I9 and delivery pipes 23. When the acid has been fed into the tank, the temperature of the batch is raised, for example by injecting steam, to the boiling point, and so maintained for about 20 minutes after all the acid has been introduced. A sample of the resultant basic calcium arsenate product is Withdrawn, analyzed for total arsenic, and the necessary adjustment made by adding hydrated lime or arsenic acid to yield a product containing about 42% total As205. The slurry is thereafter dried and ground.

During addition of the acid, the agitators f4 and I4 are rotated in the direction of arrow 32, Fig..2, at a speed of about R. P. M. A The mixing Atank used Vmay be about 'l1/2 feet in diameter and the total batch fills the tank Vto a depth of about 41/2 feet. The size of the agitators relative to the tank and the size of the vertical and horizontal baffles may be about in the same proportion as shown in the drawing. The horizontal baffles are preferably located at a level justbeneath the level of the reaction mass after all of the ingredients have been added. When operating with the batch described in apparatus of the size and arrangement indicated, driving the agitator at the speed noted and introducing the acid in the form of relatively nelms beneath the surface of the slurry, turbulent agitation as distinguished from a swishing motion is obtained, local excess of arsenic acid over that requiredto react with the calcium hydroxide is substantially prevented, and rapid diffusion of the arsenic acid is effected with the result of substantially reducing formation of the Water soluble acid calcium arsenates `to form a product of the invention containing about 1.1% Water soluble A5205, as determined by a hereinafter described method of analysis.

It will be understood that other Ways of introducing the acid into the slurry may be employed.l For example, the acid may be fed in the form of one or more preferably fine streams onto the surface of the slurry While vigorously `agitating the latter, the agitation being turbun lent in character as distinguished from a nonturbulent more or less swirling motion. The acid may also be fed into the inlet side of a centrifugal pump while transferring the slurry from one tank to another or from the bottom of a tank to the top of the same tank. The degree Vof turbulent agitation depends upon factors such as the rate of addition of acid and the size of the acid stream fed into the slurry. For example, where the acid is fed slowly into the slurry in one or more relatively fine streams, the degree of agitation may be much less than in a case where theacid is fed into the slurry more or less rapidly in larger streams. Depending upon e specific conditions, agitation should be such as to maintain the slurry in a relatively turbulent condition, atleast inthe zone of additionof the acid, sufficient to substantially avoid any local excess of arsenic acid over that required to react withv the lime, to effect rapid diffusion of the arsenic vacid throughout theY slurry, and to provide contacto'f acid with sufficient calcium hydroxide to prevent formation of water soluble calcium arsenates. Considering `any given operating conditions, the degree of agitation necesmerged in vthe. calcium sary to accomplish the purposes of the invention may be determined by one or more trial runs.

Products made by thepresent methods may be conveniently analyzed for water soluble arsenic bythe following method. To 2 grams of powdered product in a flask, add 1 liter of recently boiled Water cooled to 32 C. Stopper the flask and keep at 32 C. for 24 hours, shaking` once every hour for the first 8 hours of this period. Filter, and determine the arsenic in the filtrate which arsenic is expressed as per cent of water soluble As2O5 in the sample. The insoluble material filtered olf from the rst 24 hour digestion is Asubjected to three successive similar 24 hour digestions, each time with a fresh lot of Water, and the arsenic of each filtrate determined as before. minations should not exceed about 1.50% water soluble A5205. The example below is the full analysis of the employing varying degrees of agitation. Y

Water soluble As205% lst 24 2nd 24 3rd 24 4th 24 hours hours hours hours Example 1 nil 0. 17 0.39 0. 54 Example 2... nil O. 15 0. 29 0. 45 Example 3..- nil nil 0. 17 0. 17 Example 4...-. nil 0. 44 0. 36 0. 36 Example 5 0. 09 0. 23 0. 32 0. 35

soluble arsenic contained in a given sample andV the sum `of the four extractions is preferable. In`

general, the invention contemplates production of calcium arsenate showing not more thanabout 1.50% Water soluble the four digestions b od are totaled.

I claim:

the hereindescribed meth- In the manufacture of calcium arsenate by the addition of an aqueous arsenic acid solutionto an aqueous slurry of calcium hydroxide, the Vim-V provement which comprises gradually.` introducing the arsenic acidf'solution below the sur- Y face of the Vslurry and .adjacent to the periph-1 eryof a rapidly` rotating agitating rotor 'subhydroxide slurry so` as to effect substantially instantaneous diffusion of the arsenic acid solution through the slurry and regulating the addition of arsenic acid ,solution so that the molar ratio of arsenate to calcium in the resultant diffused mixture ldoes not exceed such ratio in basic calcium arsenate, whereby a calcium arsenate is directly produced capable of application to foliage without causing substantial burning thereof. f 5

ARTHUR H. HENNINGER.

'I'he sum of the four deter-y 1 product madev as above detailed., The products of the other examples were made by other procedures substantially as described method, any given However;

As2O5 when the results of` 

